Open source note taking and to-do app with synchronization
sleek is a todo.txt manager, free and open-source (FOSS)
WeekToDo is a Free and Open Source Minimalist Weekly Planner and To Do
IGSuite is a web-ajax-based Integrated Groupware Suite
a free mind mapping tool, to especially assist decision making
Open source project management
Collabtive is web based project management software. Simple and easy.
PHP & MySQL/SQLite -based Project Management System
Open Source, tasks, bug tracking, agile scrum, timesheet, wiki
Manage your tasks and improve your productivity
A modular ASP.NET/db4o based application framework
A trading platform including ERP, CRM, and accounting